Cometary Gas Research Articles

A Discrete Model for MHD Incorporating the Hall Term

Medium frequency plasma behavior can be investigated using an MHD model with the addition of the Hall term to the Ohm's law. Such a model embodies some electron dynamics and can therefore be used to investigate physically more complicated processes than a conventional MHD model can. Form the modeling standpoint, this model works on the ion time scale and therefore avoids the shorter electron time scale associated with a two-fluid approach. Here, we present one such model with its two as well as three-dimensional versions, test it thoroughly against the linear analytic theory, and finally present an application of the model to the study of the plasma waves generated at a solar wind cometary gas interface.

Spectrophotometry of the continuum in 18 comets

We have studied the continuum emission in spectra of 18 comets. We find that the gas-to-dust ratio (GDR) increases with increasing activity (as measured by the production of CN). This may be explained by having dust production depend on the gas production raised to the 0.55 ± 0.17 power (1σ error). We interpret this to indicate that cometary gas production in the range of heliocentric distances (and gas production rates) for which we have data, is primarily through a porous mantle. Approximately equal numbers of comets show changes in continuum brightness with distance from the optocenter equal to, or significantly slower than, the canonical (distance) −1 falloff. All continua were red compared to the solar spectrum, with an average reddening of 22% per 1000 Å (between 4400 and 5700 Å). There is some evidence for a change in the slope of the continuum with distance from the optocenter in some comets. Trends in these observables throughout the apparition of each comet are discussed, and comparisons among comets are made.

CoMA ? An advanced space experiment forin situ analysis of cometary matter

Exploring one of the most pristine bodies in our solar system — a comet — with a spacecraft will be a great step towards a deeper understanding of our solar system's beginnings. We here present the advanced space experiment CoMA (cometarymatteranalyzer), which will be flown on NASA's cometary rendezvous and asteroid flyby mission CRAF. CoMA is a high resolution time-of-flight secondary ion mass spectrometer. It will analyze μm-sized cometary dust grains and cometary gases with an unprecedented mass resolution and will yield data about the elemental, isotopic, and molecular composition.

Negative ions in the coma of comet Halley

IN March 1986, the Giotto spacecraft encountered comet Halley, approaching to within ∼600 km of the nucleus. Results from this encounter have shown that the inner coma contains a mixture of cometary neutral gas and dust, thermal ions and electrons, fast cometary pick-up ions, and decelerated solar-wind ions and electrons, as well as fast neutrals1produced by charge exchange between pick-up ions and cold neutrals. Here we report the detection of a new component of the inner coma of comet Halley: negatively charged cometary ions. These ions are observed in three broad mass peaks at 7–19, 22–65 and 85–110 AMU, with densities reaching ≳1, ∼5x10-2 and ∼4x10-2cm-3, respectively, at a distance of ∼ 2,300 km from the nucleus. The ion species thought to be present include O-, OH-, C-, CH-, CN- and heavier complex CHO molecular ions. As negative ions are easily destroyed by solar radiation at ∼1 AU (ref. 2) an efficient production mechanism, so far unidentified, is required to account for the observed densities. The detection of negative ions in the coma near 1 AU implies that in similar neutral gas and dust environments farther away from the Sun (in Jupiter's or Saturn's magnetospheres3, for example), negative ions should also be present. If the negative-ion densities are large enough, they could play an important part in physical processes such as radiative transfer or charge exchange.

The production rate and spatial distribution of H2O for Comet P/Halley

Spectroscopic observations of the O I 1D lines for Comet P/Halley are used to derive water-production rates; 43 exposures during the period August 1985 to June 1986 are reduced. The reduction procedure involves sky subtraction, ratios with solar analog comparison stars, subtraction of contaminating NH2 emissions, and absolute flux calibration. The flux collected within the 2.5-arcsec-wide x 151-arcsec-long integration aperture is converted to a total comet O I 6300-A luminosity using a cometary gas outflow model. Water-production rates for 10 preperihelion dates and seven postperihelion dates are derived. An error budget yields an estimated uncertainty of + 30 percent in H2O production rates with a maximum error of + or - 50 percent. For the preperihelion period, a good fit of the water-production rate as a function of heliocentric distance can be obtained as Q(H2O) = 4.36 x 10 to the 29th x r exp -2.54. A postperihelion enhancement by about a factor of 1.8, which appears to dissipate by June 1986, is observed. Several values of enhanced water production agree well with the observed light curve of P/Halley. Comparison of H2O production rates with those deduced by other methods shows quite reasonable agreement.

Laboratory simulation of cometary neutral gas ionization

The laboratory simulation of the interaction of the solar wind with a comet is used to study the cometary neutral gas ionization. The experiment is carried out in the UCR T‐1 facility with an ice ball as the comet model. Photographs and data are taken with a variety of values of the solar wind velocity, interplanetary magnetic field (IMF), and comet configurations. The results show that the cometary neutral gas ionization depends on both the velocity of the solar wind and the interplanetary magnetic field. The plasma cloud surrounding the comet is visible only when the solar wind velocity and IMF are both above certain minimum values. This velocity dependent phenomena is explained by Alfvén's critical ionization velocity effect. The critical magnetic field may be explained by assuming two stream lower hybrid instability as a triggering mechanism for the ionization of the neutral gas by plasma flow. Critical upper and lower limits for the magnetic field, required by anomalous ionization, are also derived that satisfy the experimental observations.

A Review of Radio Interferometric Imaging of Comets

AbstractWe present an overview of cometary observations carried out with the VLA. These include OH imaging observations, studies of other molecules and searches for continuum emission. Primarily, we discuss the OH observations obtained of Comet Halley, a periodic comet, and Comet Wilson, a “new” comet. The emission from Comet Halley is confined to a region a few times 105km. The emission from Comet Wilson shows sporadic blobs at large distances (∼ 106km from the center), which vary abruptly in both position and velocity. The generally accepted picture of excitation by UV pumping in a spherical outflow of cometary gas explains the coarse phenomenology, but other effects seem to cause significant modulations of the OH emission. We suggest that for Comet Halley, collisional effects near the edge of the quenching region, which is coincidently the approximate distance to the cometopause, may be important, and that for Comet Wilson, outgassing from cometesimals ejected from the nucleus may be significant. We further show that asymmetric line profiles are more common than previously thought. Lengthening integration times and lowering the velocity resolution results in a smooth, round shape for the comet’s brightness distribution. In addition, spectral profiles become broader and more symmetric after long integration times and after integration over larger spatial regions. In addition to the OH observations we summarize the H2CO detection experiments for comets Halley and Machholz.

A three‐dimensional MHD simulation of the interaction of the solar wind with comet Halley

A three‐dimensional time‐dependent magnetohydrodynamic (MHD) simulation was used to model the interaction between the solar wind and the plasmas produced from cometary gases. The model, which includes cometary mass loading, reproduced several features observed by the recent missions to comet Halley: (1) a weak bow shock was formed 3.2×105 km in front of the comet, (2) the magnetic field increased by a factor of 3.7 across this weak bow shock and continued to increase up to the contact surface (Bmax/BIMF = ∼6 ‐ 8), and (3) the plasma temperature increased across the bow shock but decreased nearer to the comet. In addition, interplanetary magnetic field (IMF) lines were hung up on the comet and formed a long tail in which the lobe magnetic field was quite strong (Blobe/BIMF ∼ 5). A cold dense plasma sheet oriented normal to the IMF direction formed in the tail.

Cometary gas and plasma flow with detailed chemistry

This paper describes recent hydrodynamic and magnetohydrodynamic (MHD) simulations for the gas and plasma flow around a comet with detailed photo and chemical reaction network of 59 neutral and 76 ionized chemical species. The method allows for a separate energy balance of the electrons, separate flow of neutral gas, fast neutral atomic and molecular hydrogen, and plasma with momentum exchange by elastic collisions, mass-loading of the plasma flow by ion pick-up, and Lorentz-forces of the advected magnetic field. The contact surface or magnetopause is resolved. The simulation is applied to comet Halley at the time of the Giotto encounter. The results can be compared with the data from the Giotto mission as they become available. The chemical composition of the nucleus can be inferred from these data by iteration.

Monte Carlo particle-trajectory models for neutral cometary gases. I - Models and equations. II - The spatial morphology of the Lyman-alpha coma

The mathematical derivations of various methods employed in the Monte Carlo particle-trajectory model (MCPTM) are presented, and the application of the MCPTM to the calculation of the photochemical heating of the inner coma through the partial thermalization of cometary hydrogen atoms produced by the photodissociation of water is discussed. This model is then used to explain the observed morphology of the spatially extended Ly-alpha comas of comets. The rocket and Skylab images of the Ly-alpha coma of Comet Kohoutek are examined.

The Giotto Implanted Ion Spectrometer (IIS): principles of the electronic operation and data collection

The Implanted Ion Spectrometer (IIS) is part of the solar wind ion plasma analyser JPA in the payload of the European Giotto mission to Comet Halley (1986 III). The IIS complements the observation of the solar wind bulk parameter by its focus on the acceleration of ionised cometary gases and its interaction with the solar wind plasma. The state of the art low-power/low-weight signal conditioning unit (SCU) of the implanted ion spectrometer features several new concepts. A novel FINDER unit provides the possibility to operate all sensors of the multisensor spectrometer simultaneously, thus avoiding adverse duty cycle effects. A complete functional test of all major IIS components and IIS related elements in the DPU is obtained from a new scheme for in-flight calibration. Functional principles of the key components in the SCU which have significance for the data analysis are described.

Gasdynamic interpretation of ice measurements at the Giacobini-Zinner encounter

Plasma measurements of the ICE spacecraft at its encounter with comet Giacobini-Zinner are found to be in qualitative agreement with corresponding predictions of a gasdynamic model on the solar wind interaction with the cometary neutral gas.

Activity of comet Halley observed in the ultraviolet

The time variation of the water production rate of comet Halley during this apparition has been deduced from observations of the Ultraviolet Imager on board Suisei. The preperihelion production rates were a factor of 2 ∼ 3 smaller than postperihelion rates, suggesting a postperihelion heating of the cometary nucleus. Some fine intensity structure in the Lyman alpha photometry data taken at encounter, which indicates spatial variation of the atomic hydrogen density in the coma, may correspond to shell structures in the Lyman alpha images so far obtained and is ascribed to the presence of a hydrogen source other than water in the cometary gas or dust particles. The velocity of these hydrogen atoms is estimated to be about 11 km s−1.

Critical ionization velocity effects in the inner coma of comet Halley: Measurements by Vega‐2

Plasma density and plasma wave measurements aboard the "Vega‐2" spacecraft discovered two plasma density enhancements in the inner coma of comet Halley that are accompanied by bursts of plasma waves in the lower‐hybrid frequency range. These events are explained by the critical ionization velocity (CIV) effects of Alfvén. The persistent bursty plasma wave activity and the intermittent plasma density growth toward the cometary nucleus indicate that the CIV effect contributes to the ionization of the cometary gas.

Simulation of ICE-Giacobini-Zinner and VEGA/GIOTTO-Halley encounters

The formation of a weak shock at the interaction of the solar wind with the cometary neutral gas is confirmed in a non-stationary gasdynamic model. No shocks will be detectable along the ICE path and if the VEGA/GIOTTO spacecraft enter the cometary atmosphere. A shock can only be expected at an exit from the Halley atmosphere.

Structures in the coma of comet Halley

Dense dust jets are expected to be encountered by the spacecraft penetrating the coma of comet Halley in March 1986. At the time of encounters, remote observations of the global distribution of coma constituents will complement the detailed in situ measurements. Here we report ground-based measurements of the optical brightness distribution of the coma, made in November 1985, when the comet was at a distance of 1.75 AU from the Sun. From the total brightness we deduce a dust production rate of 1.5 × 106 g s−1, but our results show that cometary dust and gas production, which seem to be correlated, are strongly anisotropic and time-variable. Structures in the coma can be revealed by comparison with a physical model of the coma, but astrometry of the comet nucleus is adversely affected by the anisotropic brightness distribution in the coma.

Neutral gas measurements of comet Halley from Vega 1

The neutral gas experiment (ING) aboard the Vega 1 spacecraft detected cometary gas within a distance of 60,000 km from the nucleus of comet Halley. A preliminary inspection of the data permits an analysis of the variation in neutral gas density. Fine structure was detected in the spatial distribution of lower-mass species.

A model of cometary gas and dust production and nongravitational forces with application to P/Halley

Abstract A program that computes gas and dust production rates and idealized nongravitational force components has been developed and applied to the case of Comet Halley. We use a modified form of our earlier comet model (F.P. Fanale and J.R. Salvali[(1984) Icarus 60 , 476–511] to which coma effects and a section on nongravitational forces have been added. The possibility of grain cohesion is also included. These models are used together with observations from 1910 and semiempirically derived data to investigate the effects of obliquity and thermal conductivity of the near thermal conductivity of the nucleus on gas and dust production. The results indicate that the thermal conductivity of the nucleus is of the order of 10 5 ergs/cm-s-°K, which implies that the ice near the surface is in the crystalline form. A general method is presented for calculating the radii of cometary nuclei using theoretically derived and semiempirically derived nongravitational force components. This method is used to calculate possible radii for Comet Halley that depend on the model variation chosen. The method used and the results presented herein should have greater significance and value when the observational data from Halley's current perihelion passage become available.

Time-dependent dusty gasdynamical flow near cometary nuclei

This paper presents time-dependent solutions to the coupled dusty hydrodynamics equations describing the spherically symmetric expansion of cometary neutral gas in the vicinity of a cometary nucleus. The sublimation process is repressented by gas outflow from a dust-covered reservoir containing stationary gas whose pressure and density values are determined by the sublimating (Ts) and surface (T0) temperatures. The model resolves earlier ambiguities in determining gas production rates and provides analytic relations between Ts, T0, and the gas parameters at the sonic point. The time evolution of a cometary outburst was modeled. It was found that, as a result of the strong gas-dust interaction in the inner coma region, a 'slow' disturbance in both the dust and gas parameters will be created in addition to the familiar gas blast-wave solution. This new 'slow' disturbance, which propagates with a velocity of about 0.2 km/s, might be responsible for some of the observed slowly expanding cometary halos, such as the one which was recently identified using 1910 Mount Wilson high-resolution comet Halley photographs.

Halley's comet mission

Halley's comet will cross the earth's orbit and will remain within reasonable distance for spacecraft observation during the period November 1985 through March 1986. The European Space Agency (ESA) will be sending the Giotto spacecraft to study Halley. There is about a 70/40 proportion of European to American scientists on the mission team.Out of 10 instruments to observe the coma on board Giotto as it passes through Halley's cometary dust at a relative velocity of 244,800 km/hour, two mass spectrometers will sample and analyze cometary gases. One of these spectrometers is being built in part by the Lockheed Missiles and Spacecraft Company's Research Laboratory, and the other one is being built by the Max Planck Institute in Landau, Federal Republic of Germany. If the system survives bombardment by particles, the gas composition of a comet will be known for the first time.